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From:   Guo Ren <guoren@kernel.org>
Date:   Fri, 21 Oct 2022 22:35:23 +0800
Message-ID: <CAJF2gTROm_ifvbccUmj0_CTLNq5oh1GC+XCLX+NQm4L5P71CFw@mail.gmail.com>
Subject: Re: [PATCH] riscv: fix race when vmap stack overflow
To:     Andrea Parri <parri.andrea@gmail.com>
Cc:     Jisheng Zhang <jszhang@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
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On Fri, Oct 21, 2022 at 4:36 PM Andrea Parri <parri.andrea@gmail.com> wrote:
>
> > > > +     atomic_set_release(&spin_shadow_stack, 0);
> > >
> > > Have not really looked the details: should there be a matching acquire?
> >
> > I use atomic_set_release here, because I need earlier memory
> > operations finished to make sure the sp is ready then set the spin
> > flag.
> >
> > The following memory operations order is not important, because we
> > just care about sp value.
> >
> > Also, we use relax amoswap before, because sp has naturelly
> > dependency. But giving them RCsc is okay here, because we don't care
> > about performance here.
>
> Thanks for the clarification.
>
> I'm not really suggesting to add unneeded synchronization, even more
> so in local/private constructs as in this case.  It just felt odd to
> see the release without a pairing acquire, so I asked.  ;-)
Okay, let's keep:

 handle_kernel_stack_overflow:
+1:     la sp, spin_shadow_stack
+       amoswap.w sp, sp, (sp)
+       bnez sp, 1b
+
....
+     smp_store_release(&spin_shadow_stack, 0);

>
> Thanks,
>   Andrea
>
>
> > eg:
> >  handle_kernel_stack_overflow:
> > +1:     la sp, spin_shadow_stack
> > +       amoswap.w.aqrl sp, sp, (sp)
> > +       bnez sp, 1b
> > +
> > ....
> > +     smp_store_release(&spin_shadow_stack, 0);
> > +     smp_mb();


-- 
Best Regards
 Guo Ren